SNAB Unit 2 Revision

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3. Organelles


A large organelle surrounded by a nuclear envelope (double membrane), which contains many pores. The nucleus contains chromatin (which is made from DNA and proteins) and a structure called the nucleolus

The nucleus controls the cell's activities (by controlling the transcription of DNA). DNA contains instructions to make proteins. The pores allow substances (e.g. RNA) to move between the nucleus and the cytoplasm. The nucleolus makes ribosomes


A round organelle surrounded by a membrane, with no clear internal structure. 

Contains digestive enzymes. These are kept separate from the cytoplasm by the surrounding membrane, and can be used to digest invading cells or to break down worn out components of the cell. 

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3. Organelles


A very small organelle that either floats free in the cytoplasm or is attached to the rough endoplasmic reticulum. It's made up of proteins and RNA. It's not surrounded by a membrane.

The site where proteins are made.  

Rough Endoplasmic Reticulum (rER):

A system of membranes enclosing a fluid-filled space. The surface is covered with ribosomes

Folds and processes proteins that have been made at the ribosomes. 

Smooth Endoplasmic Reticulum (sER):

Similar to rER, but with no ribosomes.

Synthesises and processes lipids

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3. Organelles

Golgi apparatus:

A group of fluid-filled, membrane-bound, flattened sacs. Vesicles are often seen at the edges of the sacs.

It processes and packages new lipids and proteins. It also makes lysosomes


They have a double membrane- the inner is folded to form structures called cristae. Inside is the matrix, which contains enzymes involved in respiration. 

The site of aerobic respiration, where ATP is produced.


Small, hollow cylinders, made of microtubules

Involved with separation of chromosomes during cell division. 

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3. Protein Production and Transport

1) Proteins are made at the ribosomes.

2) The ribosomes on the rER make proteins that are excreted or attached to the cell membrane. The free ribosomes in the cytoplasm make proteins that stay in the cytoplasm

3) New proteins produced at the rER are folded and processed (e.g. sugar chains are added) in the rER.

4) Then they're transported from the rER to the golgi apparatus in vesicles

5) At the golgi apparatus, the proteins may undergo further processing (e.g. sugar chains are trimmed or more are added)

6) The proteins enter more vesicles to be transported around the cell. E.g. extracellular enzymes (like digestive enzymes) move to the cell surface and are secreted

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3. Prokaryotic Cells

The cytoplasm has no membrane-bound organelles. It has ribosomes- but they're smaller than those in a eukaryotic cell. 

The plasma membrane is mainly made of lipids and proteins. It controls the movement of substances into and out of the cell. 

The cell wall supports the cell and prevents it form changing shape. 

Some prokaryotes have short hair-like structures called pili. Pili help prokaryotes stick to other cells and can be used in the transfer of genetic material between cells. 

Some prokaryotes also have a slime capsule. It helps to protect bacteria from attack by cells in the immune system. 

Mesosomes are inward folds in the plasma membrane. Scientists are still debating their function

Plasmids are small loops of DNA that aren't part of the main circular DNA material. 

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3. Prokaryotic Cells

Unlike a eukaryotic cell, a prokaryotic cell doesn't have a nucleus. Instead, the DNA floats free in the cytoplasm. It's circular DNA, present as one long coiled-up strand. It's not attached to any histone proteins.

The flagellum is a long, hair-like structure that rotates to make the prokaryotic cell move. Not all prokaryotes have a flagellum. Some have more than one

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3. Cell organisation

Similar cells are organised into tissues.

A tissue is a group of similar cells that are specially adapted to work together to carry out a particular function. 

Tissues are organised into organs.

An organ is a group of different tissues that work together to perform a particular function. 

Different organs make up an organ system.

Organs work together to form organ systems- each system has a particular function

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3. Cell Cycle

In mitosis, a parent cell divides to produce two genetically identical daughter cells (they contain an exact copy of the DNA of the parent cell).

Mitosis is needed for the growth of multicellular organisms, for repairing damaged tissues and for asexual reproduction (reproduction from just one parent).

In multicellular organisms, not all cells keep their ability to divide. The ones that do, follow a cell cycle

Mitosis is part of the cell cycle:

The cell cycle consists of a period of cell growth and DNA replication called interphase. Mitosis happens after that. Interphase is subdivided into three separate growth stages. 

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3. Mitosis

Interphase- The cell carries out normal functions, but also prepares to divide. The cell's DNA is unravelled and replicated, to double its genetic content. The organelles are also replicated so it has spare ones, and its ATP content is increase. 

1) Prophase- The chromosomes condense, getting shorter and fatter. Tiny bundles of protein called centrioles start moving to opposite ends of the cell, forming a network of protein fibres across it called the spindle. The nuclear envelope breaks down and chromosomes lie free in the cytoplasm. 

2) Metaphase- The chromosomes line up along the middle of the cell and become attached to the spindle by their centromere

3) Anaphase- The centromeres divide, separating each pair of sister chromatids. The spindles contract, pulling chromatids to opposite poles of the spindle, centromere first. 

4) Telophase- The chromatids reach the opposite poles on the spindle. They uncoil and become long and thin again. They're now called chromosomes again. A nuclear envelope forms around each group of chromosomes, so there are now two nuclei. The cytoplasm divides and there are now two daughter cells that are genetically identical to the original cell and to each other. 

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3. Root tip experiment

1) Cut 1cm from the tip from a growing root (e.g. an onion). It needs to be the tip because that's where growth occurs (and so that's where mitosis takes place).

2) Prepare a boiling tube containing 1M hydrochloric acid and put it in a water bath at 60 degrees. 

3) Transfer the root tip into the boiling tube and incubate for about 5 minutes

4) Use a pipette to rinse the root tip well with cold water. Leave the tip to dry on a paper towel

5) Place the root tip on a microscope slide and cut 2 mm from the very tip of it. 

6) Use a mounted needle to break the tip open and spread the cells out thinly. 

7) Add a small drop of stain and leave it for a few minutes. The stain will make the chromosomes easier to see under a microscope. 

8) Place a cover slip over the cells and push down firmly to squash the tissue. This will make the tissue thinner and allow light to pass through it. 

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3. Gametes and Fertilisation

Gametes are the male and female sex cells found in all organisms that reproduce sexually.

They join together at fertilisation to form a zygote, which divides and develops into a new organism

In animals, the male gametes are sperm and the female gametes are egg cells

Normal body cells contain the full number of chromosomes. Humans have two sets of 23 chromosomes- one set from the male parent and one from the female parent- giving each body cell a total of 46 chromosomes

Gametes contain half the number of chromosomes as body cells- they only contain one set.

Since each gamete contains half the full number of chromosomes, fertilisation creates a zygote with the full number of chromosomes. Fertilisation is the term used to describe the exact moment when the nuclei of the male and female gametes fuse

Combining genetic material from two individuals makes offspring that are genetically unique

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3. Gametes and Fertilisation

Egg cells and sperm cells have all the same organelles as other eukaryotic cells, including a nucleus and a cell membrane. The structures of egg and sperm cells are also specialised for their function- bring the female and male DNA together at fertilisation to form a zygote

Egg Cell:

Egg cells are much larger than sperm. Egg cells also contain huge food reserves to nourish the developing embryo. Follicle cells form protective coating. Zona Pellucida- protective glycoprotein layer that sperm have to penetrate. 

Sperm Cell:

Respiration takes place in mitochondria. This releases energy in the form of ATP, which sperm use to swim. Lots of mitochondria provide energy for tail movement. Acrosome contains digestive enzymes to break down the egg cell's zona pellucida and enable sperm to penetrate the egg. Flagellum allows sperm to swim towards egg cell. 

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3. Gametes and Fertilisation

In mammals, sperm are deposited high up in the female ****** close to the entrance of the cervix. Once there, they have to make their way up through the cervix and uterus, and into one of the oviducts. Once the sperm are in the oviduct, fertilisation may occur:

1) The sperm swim towards the egg cell in the oviduct. 

2) Once one sperm makes contact with the zona pellucida of the egg cell the acrosome reaction occurs- this is where digestive enzymes are released from the acrosome of the sperm. 

3) These enzymes digest the zona pellucida, so that the sperm can move through it to the cell membrane of the egg cell. 

4) The sperm head fuses with the cell membrane of the egg cell. This triggers the cortical reaction- the egg cell releases the contents of the vesicles called cortical granules into the space between the cell membrane and the zona pellucida.

5) The chemicals from the cortical granules make the zona pellucida thicken, which makes it impenetrable to other sperm. Only the sperm nucleus enters the egg cell. The nucleus of the sperm fuses with the nucleus of the egg cell- this is fertilisation

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3. Meiosis

1) The DNA replicates so there are two identical copies of each chromosome, called chromatids

2) The DNA condenses to form double-armed chromosomes, made from two sister chromatids

3) The chromosomes arrange themselves into homologous pairs- pairs of matching chromosomes.

4) First division- the homologous pairs are separated, halving the chromosome number. 

5) Second division- the pairs of sister chromatids are separated.

6) Four new daughter cells that are genetically different from each other are produced. These are gametes

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3. Crossing Over of Chromatids

1) Before the first division of meiosis, homologous pairs of chromosomes come together and pair up.

2) Two of the chromatids in each homologous pair twist around each other. 

3) The twisted bits break off their original chromatid and rejoin onto the other chromatid, recombining their genetic material. 

4) The chromatids still contain the same genes but they now have a different combinations of alleles

5) This means that each of the four new cells formed from meiosis contains chromatids with different alleles

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3. Independent assortment of chromosomes

1) The four daughter cells formed from meiosis have completely different combinations of chromosomes

2) All your cells have a combination of chromosomes from your parents, half from your mum (maternal) and half from your dad (paternal). 

3) When the gametes are produced, different combinations of those maternal and paternal chromosomes go into each cell. 

4) This is called independent assortment (separation) of the chromosomes. 

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3. Inheritance

The position of a gene on a chromosome is called a locus. Independent assortment means that genes with loci on different chromosomes end up randomly distributed in the gametes

But genes, with loci on the same chromosome are said to be linked- because the genes are on the same chromosome, they'll stay together during independent assortment and their alleles will be passed on to the offspring together. The only alleles will be passed on to the offspring together. The only reason this won't happen is if crossing over splits them up first. 

The closer together the loci of two genes on a chromosome, the more closely they are said to be linked. This is because crossing over is less likely to split them up. 

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3. Inheritance

A characteristic is said to be sex-linked when the locus of the allele that codes for it is on a sex chromosome. 

In mammals, females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).

The Y chromosome is smaller than the X chromosome and carries fewer genes. So most genes on the sex chromosomes are only carries on the X chromosome (called X-linked genes).

As males only have one X chromosome, they often only have one allele for sex-linked genes. So because they only have one copy, they express the characteristic of the allele even if it's recessive. This makes males more likely then females to show recessive phenotypes for genes that are sex-linked. 

Genetic disorders caused by faulty alleles on sex chromosomes incluude colour blindless and haemophilia. The faulty alleles for both of these disorders are carried on the X chromosome- they're called X-linked disorders. 

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3. Cell Differentiation

Multicellular organisms are made up from many different cell types that are specialised for their function, e.g. liver cells, muscle cells, white blood cells. All these specialised cell types originally came from stem cells

Stem cells are unspecialised cells that can develop into other types of cell. Stem cells divide by mitosis to become new cells, which then become specialised. The process by which a cell becomes specialised is called differentiation

All multicellular organisms have some form of stem cell. In humans, some stem cells are found in the embryo. The ability of stem cells to differentiate into specialised cells is called potency

Totipotency- the ablility to produce all cell types, including all the specialised cells in an organism. Pluripotency- the ablility of a stem cell to produce all the specialised cells in an organism. 

Totipotent stem cells are only present in mammals in the first few cell divisions of an embryo. After this point the embryonic stem cells become pluripotent

Stem cells are also found in some adult tissues. Adult stem cells are much less flexible than embryonic stem cells- they can only develop into some cell types. 

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3. Cell Differentiation

A cell's genome is its entire set of DNA, including all the genes it contains. However, a cell doesn't express (make proteins from) all the genes in its genome. Stem cells become specialised because different genes in their DNA become active and get expressed:

Stem cells all contain the same genes, but not all of them are expressed because not all of them are active

Under the right conditions, some genes are activated and others are inactivated

mRNA is only transcribed from the active genes

The mRNA from the active genes is then translated into proteins.

These proteins modify the cell- they determine the cell structure and control cell processes (including the activation of more genes which produces more proteins). 

Changes to the cell produced by these proteins cause the cell to become specialised. These changes are difficult to reverse, so once a cell has specialised it stays specialised. 

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3. Gene Expression

Gene expression can be controlled by altering the rate of transcription of genes. This is controlled by transcription factors- proteins that bind to DNA and activate and deactivate genes by increasing or decreasing the rate of transcription. 

Factors that increase the rate of transcription are called activators and those that decrease the rate are called repressors. Activators often work by helping RNA polymerase bind to the DNA and begin transcription. Repressors often work by preventing RNA polymerase from binding and so stopping transcription. 

In eukaryotes, such as animals and plants, transcription factors bind to specific DNA sites near the start of their target genes- the genes they control the expression of. In prokaryotes, control of gene expression often involves transcriptin factors binding to operons. 

An operon is a section of DNA that contains a cluster of structural genes, that are transcribed together as well as control elements and sometimes a regulatory gene:                                      The structural genes code for useful proteins, such as enzymes. The control elements include a promoter (a DNA sequence located before the structural genes that RNA polymerase binds to) and an operator (a DNA sequence that transcription factors bind to). The regulatory gene codes for an activator or repressor. 

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3. Gene Expression

E.coli is a bacterium that respires glucose, but it can use lactose if glucose isn't available. 

The genes that produce the enzymes needed to respire lactose are found on an operon called the lac operon

The lac operon has three structural genes- lacZ, lacY and lacA, which produce proteins that help the bacteria digest lactose (including B-galactosidase and lactose permease). 

Lactose NOT present:                                                                                                                         The regulatory gene produces the lac repressor, which is a transcription factor that binds to the operator site when there's no lactose present. This blocks transcription because RNA polymerase can't bind to the promoter. 

Lactose present:                                                                                                                                 When lactose is present, it binds to the repressor, changing the repressor's shape so that it can no longer bind to the operator site. RNA polymerase can now begin transcription of the structural genes. 

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3. Stem Cells

Stem cells can develop into any specialised cell type, so scientists think they could be used to replace damaged tissues in a range of diseases

Some stem cell therapies already exist

Scientists are researching the use of stem cells as a treatment for lots of conditions, including:     Spinal cord injuries; heart disease and damage caused by heart attacks.

People who make decisions about the use of stem cells in medicine and research have to consider the potential benefits of stem cell therapies:

- They could save many lives

- They could improve the quality of life for many people. 

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3. Stem Cells

Adult Stem Cells:

- Obtained from the body tissues of an adult. For example, adult stem cells are found in bone marrow

- They can be obtained in a relatively simple operation- with very little risk involved, but quite a lot of discomfort. The donor is anaesthetised, a needle is inserted into the centre of the bone and a small quantity of bone marrow is removed

- Adult stem cells aren't as flexible as embryonic stem cells- they can only develop into a limited range of cells. 

- However, if a patient needs a stem cell transplant and their own adult stem cells can be used there's less risk of rejection

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3. Stem Cells

Embryonic Stem Cells:

- These are obtained from early embryos.

- Embryos are created in a laboratory using in vitro fertilisation (IVF)- egg cells are fertilised by sperm outside the womb

- Once the embryos are approximately 4 to 5 days old, stem cells are removed from them and the rest of the embryo is destroyed.

- Embryonic stem cells can develop into all types of specialised cells. 

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3. Stem Cells

Obtaining stem cells from embryos created by IVF raised ethical issues because the procedure results in the destruction of an embryo that's viable (could become a fetus if placed in the womb).

Many people believe that at the moment of fertilisation a genetically unique individual is formed that has the right to life- so they believe that it's wrong to destroy embryos. 

Some people have fewer objections to stem cells being obtained from egg cells that haven't been fertilised by sperm, but have been artifically activated to start dividing. This is because the cells couldn't survive past a few days and wouldn't produce a fetus if placed in a womb. 

Some people think that scientists should only use adult stem cells because their production doesn't destroy an embryo. But adult stem cells can't develop into all the specialised cell types that embryonic stem cells can. 

The decision-makers in society have to take into account everyone's views when making decisions about important scientific work like stem cell research and its use in medicine. 

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3. Variation

Continuous variation:

This is when the individuals in a population vary within a range- there are no distinct categories. For example:



Skin colour

Discontinuous variation:

This is when there are two or more distinct categories- each individual falls into only one of these categories. For example:

Blood group

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3. Variation

Individuals of the same species have different genotypes (different combinations of alleles). 

This variation in genotype results in variation in phenotype- the characteristics displayed by an organism. For example, in humans there are six different combinations of blood group alleles, which can produce one of four different blood groups. 

Some characteristics are controlled by only one gene- they're called monogenic. They tend to show discontinuous variation, e.g. blood group.

Most characteristics are controlled by a number of genes at different loci- they're said to be polygenic. They usually show continuous variation, e.g. height. 

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3. Variation

Some characteristics are only influenced by genotype, e.g. blood group.

Most characteristics are influenced by both genotype and the environment, e.g. weight. 

For example:

Height is polygenic and affected by environmental factors, especially nutrition. E.g. tall parents usually have tall children, but if the children are undernourished they won't grow to their maximum height (because protein is required for growth).

Monoamine Oxidase A (MAOA) is an enzyme that breaks down monoamines (a type of chemical) in humans. Low levels of MAOA have been linked to mental health problems. MAOA production is controlled by a single gene (it's monogenic), but taking anti-depressents or smoking tobacco can reduce the amount produced. 

Cancer is the uncontrolled division of cells that leads to lumps of cells (tumours) forming. The risk of developing some cancers is affected by genes, but environmental factors such as diet can also influence the risk. 

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3. Variation

Changes in the environment can cause changes in gene expression

In eukaryotes, epigenetic control can determine whether certain genes are expressed, altering the phenotype

Epigenetic control doesn't alter the base sequence of DNA. It works by attaching or removing chemical groups to or from the DNA. This alters how easy it is for enzymes and other proteins needed for transcription to interact with and transcribe genes. 

Epigenetic changes to gene expression play a role in lots of normal cellular processes. They can also occur in response to changes in the environment- e.g. pollution and availabilty of food. 

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3. Variation

Increased methylation of DNA represses a gene.

One method of epigenetic control is methylation of DNA- this is when a methyl group is attached to the DNA coding for a gene

The group always attaches at a CpG site, which is where a cytosine and guanine base are next to each other in the DNA.

Increased methylation changes the DNA structure, so that the proteins and enzymes needed for transcription can't bind to the gene- so the gene is not expressed (i.e. it's repressed or inactivated). 

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3. Variation

Modification of histones also affects gene expression.

Histones are proteins that DNA wraps around to form chromatin, which makes up chromosomes. Chromatin can be highly condensed or less condensed. How condensed it is affects the accessibilty of the DNA and whether or not the proteins and enzymes needed for transcription can bind to it. 

Epigenetic modifications to histones include the addition or removal of acetyl groups:

1) When histones are acetylated, the chromatin is less condensed. This means that the proteins involved in transcription can bind to the DNA, allowing genes to be transcribed (i.e.the genes are activated). 

2) When acetyl groups are removed from the histones, the chromatin becomes highly condensed and genes in the DNA can't be transcribed because the transcription proteins can't bind to them- the genes are repressed

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4. Biodiversity and Endemism

Biodiversity is the variety of living organisms in an area:

- Species diversity- the number of different species and the abundance of each species in an area. 

- Genetic diversity- the variation of alleles within a species (or a population of a species). 

Endemism is when a species is unique to a single place 

Natural selection, leading to adaptation and evolution has increased biodiversity on earth over time. But human activities, such as farming or deforestation, are reducing species diversity- causing biodiversity to fall as a result. 

Conservation is needed to help maintain biodiversity. It is also really important for endemic species because they're particularly vunerable to extinction. They're only found in one place, so if their habitat is threatened they can't usually migrate and their numbers will decline

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4. Biodiversity and Endemism

Diversity within a species is the variety shown by individuals of that species (or within a population of that species). 

Individuals of the same species vary because they have different alleles.

Genetic diversity is the variety of alleles in the gene pool of a species (or population).

The gene pool is the complete set of alleles in a species. 

The greater the variety of alleles, the greater the genetic diversity. For example, animals have different alleles for blood group. In humans there are three alleles for blood group, but gorillas have only one, so humans show greater genetic diversity for blood group for gorillas. 

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4. Biodiversity and Endemism


Phenotype describes the observable characteristics of an organism

Different alleles code for slightly different versions of the same characteristics. 

By looking at the different phenotypes in a population of a species, you can get an idea of the diversity of alleles in that population. 

The larger the number of different phenotypes, the greater the genetic diversity. 

For example, humans have different eye colours due to different alleles. Humans in northern Europe show a variety of blue, grey, green or brown eyes. Outside this area, eye colour shows little variety- they're usually brown. There's greater genetic diversity in eye colour in northern Europe. 

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4. Biodiversity and Endemism


Samples of an organism's DNA can be taken and the sequence of base pairs analysed.

The order of bases in different alleles is slightly different, e.g. the allele for brown hair will have a slightly different order of bases than the allele for blonde hair. 

By sequencing the DNA of individuals of the same species, you can look at similarities and differences in the alleles within a species. 

You can measure the number of different alleles a species has for one characteristic to see how genetically diverse the species is. The larger the number of different alleles, the greater the genetic diversity. 

You can also look at the heterozygosity index

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4. Adaptation and Evolution

Organisms can be adapted to their niche in three ways. Adaptations are features that increase an organism's chance of survival and reproduction


- Ways an organism acts that increase its chance of survival. For example, possums sometimes 'play dead' - if they're being threatened by a predator they play dead tp escape attack. This increases their chance of survival. 


- Processes inside an organism's body that increase its chance of survival. For example, brown bears hibernate- they lower their rate of metabolism over winter. This conserves energy, so they don't need to look for food in the months when it's scarce.


- Structural features of an organism's body that increase its chance of survival. For example, whales have a thick layer of blubber- this helps them keep warm in the cold sea. 

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4. Adaptation and Evolution

Mutations can introduce new alleles into a population, so individuals within a population show variation in their phenotypes. Some of these alleles determine characteristics that can make the individual more likely to survive

Selection pressures such as predation, disease and competition create a struggle for survival.

Individuals without the advantageous alleles don't survive. This means there are fewer individuals and less competition for resources

Individuals with better adaptations (characteristics that give a selective advantage) are more likely to survive, reproduce and pass on their advantageous alleles to their offspring

Over time, the number of individuals with the advantageous alleles increases

Over generations this leads to evolution as the frequency of the advantageous alleles in the population increase and the favourable adaptations become more common

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4. Adaptation and Evolution

Speciation is the development of a new species.

A species is defined as a group of similar organisms that can reproduce to give fertile offspring.

Speciation is the development of a new species. 

It occurs when populations of the same species become reproductively isolated- the changes  in the alleles and phenotypes of the populations prevent them from successfully breeding together. These changes include:

- Seasonal changes- individuals from the same population develop different flowering or mating seasons, or become sexually active at different times of the year. 

- Mechanical changes- changes in genitalia prevent successful mating. 

- Behavioural changes- a group of individuals 

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